Imaging device and camera system with photosensitive conversion element

ABSTRACT

An imaging device including a pixel array section functioning as a light receiving section which includes photoelectric conversion devices and in which a plurality of pixels, which output electric signals when photons are incident, are disposed in an array; a sensing circuit section in which a plurality of sensing circuits, which receive the electric signals from the pixels and perform binary determination regarding whether or not there is an incidence of photons on the pixels in a predetermined period, are arrayed; and a determination result integration circuit section having a function of integrating a plurality of determination results of the sensing circuits for the respective pixels or for each pixel group, wherein the determination result integration circuit section derives the amount of photon incidence on the light receiving section by performing photon counting for integrating the plurality of determination results in the plurality of pixels.

RELATED APPLICATION DATA

This application is a continuation of U.S. patent application Ser. No.14/701,817 filed May 1, 2015 which is a division of U.S. patentapplication Ser. No. 13/241,758 filed Sep. 23, 2011, now U.S. Pat. No.9,055,244 issued Jun. 9, 2015 the entireties of which are incorporatedherein by reference to the extent permitted by law. The presentapplication claims the benefit of priority to Japanese PatentApplication No. JP 2010-224235 filed on Oct. 1, 2010 in the Japan PatentOffice, the entirety of which is incorporated by reference herein to theextent permitted by law.

FIELD

The present disclosure relates to an imaging device, such as a CMOSimage sensor, and a camera system.

BACKGROUND

Measurement or imaging of minute luminescence or fluorescence emittedfrom the body has become increasingly active in the field of medicine orbiotechnology in recent years.

In the medical or security field, a technique of converting a smallquantity of X-rays transmitted through the body into visible-levelphotons through a scintillator and detecting them to perform atransmission imaging has been industrialized. In addition, in themedical or security field, a technique (for example, a SPECT or a PET)of converting γ-rays generated from a small quantity of radiationmaterial injected into the human body into photons through ascintillator has been industrialized.

In imaging in such a field, a photon counter is used for a very smallamount of light.

Typically, a photon counter is a single device using an avalanche diodeor a photomultiplier tube.

This photon counter generates a voltage pulse at the output byconverting photons incident on the light receiving surface intophotoelectrons, accelerating the photoelectrons with a high voltage, andmultiplying them by generation of secondary electrons by collision.

The number of pulses is measured by a counter device connected to thedevice all the time.

While the photon counter has high measurement accuracy allowingdetection in units of one photon, the system is expensive and thedynamic range for measurement is also narrow.

Usually, the number of photons which can be measured by one photoncounter is about 1 million to 10 million for 1 second.

On the other hand, for imaging in a range of a relatively large amountof light to be measured, a photodiode and an analog-to-digital (AD)converter are used.

The photodiode accumulates electrode charges photoelectrically convertedand outputs an analog signal. This analog signal is converted into adigital signal by the AD converter.

Problems in such imaging are noise caused by transmission of an analogsignal and the conversion rate of the AD converter.

In order to detect a small amount of light, it is necessary to suppressnoise and also to increase the number of bits in AD conversion for finechopping. However, in order to do so, a very high-speed AD converter isnecessary. In addition, if this is made to have a large number of pixelsin order to improve the resolution in imaging, the system size for ADconversion is significantly increased.

SUMMARY

Basically, both low-noise and high-accuracy optical detection and alarge dynamic range are necessary for imaging of a small amount oflight.

However, there is no device which meets both the requirements.

For example, in order to reduce the amount of exposure in X-ray imaging,the accuracy equivalent to the level of a photon counter is necessary.In a normal photon counter, however, it is not possible to obtain adynamic range sufficient for imaging.

In addition, a large number of pixels are necessary in order to improvethe resolution. In this case, however, the system including a counterdevice is very expensive.

On the other hand, JP-A-1995-67043 proposes a new photon counting methodusing time division.

This is to acquire the two-dimensional imaging data by performing binarydetermination regarding whether or not there is an incidence of a photonon a photodiode in a fixed period and integrating results obtained byrepeating the binary determination multiple times.

That is, a signal from a photodiode is sensed every fixed period, and acounter connected to each pixel is counted up by 1 regardless of thenumber of incident photons if the number of photons incident for theperiod is 1 or more.

If the frequency of photon incidence is random on the time axis, therelationship between the actual number of photon incidence and thenumber of counts follows a Poisson distribution. Accordingly, it becomesan approximately linear relationship if the incidence frequency is low,and uniform correction can be performed if the incidence frequency ishigh.

However, according to the technique disclosed in JP-A-1995-67043, theaperture area of a pixel is extremely reduced since a sensing circuitand a counter are necessary for each pixel.

JP-A-2004-193675 proposes a configuration in which counters are disposedoutside a pixel array while adopting the above-described time-divisioncounting method. However, a sensing circuit and a memory are stillnecessary for each pixel.

A counter is provided for each pixel even if the counter is providedoutside the pixel array. Accordingly, the circuit size of a chip isinevitably increased.

Moreover, in order to increase the dynamic range in imaging in theconfiguration disclosed in JP-A-1995-67043 or JP-A-2004-193675, it isnecessary to chop a measurement period of photon incidence finely on thetime axis and to increase the pixel access speed.

Thus, it is desirable to provide an imaging device and a camera systemallowing imaging or light intensity measurement with less noise even atlow illuminance and with a wide dynamic range.

An embodiment of the present disclosure is directed to an imaging deviceincluding: a pixel array section functioning as a light receivingsection which includes photoelectric conversion devices and in which aplurality of pixels, which output electric signals when photons areincident, are disposed in an array; a sensing circuit section in which aplurality of sensing circuits, which receive the electric signals fromthe pixels and perform binary determination regarding whether or notthere is an incidence of photons on the pixels in a predeterminedperiod, are arrayed; and a determination result integration circuitsection having a function of integrating a plurality of determinationresults of the sensing circuits for the respective pixels or for eachpixel group. The determination result integration circuit sectionderives the amount of photon incidence on the light receiving section byperforming photon counting for integrating the plurality ofdetermination results in the plurality of pixels.

Another embodiment of the present disclosure is directed to a camerasystem including: an imaging device; an optical system which forms asubject image on the imaging device; and a signal processing circuitwhich processes an output image signal of the imaging device. Theimaging device includes: a pixel array section functioning as a lightreceiving section which includes photoelectric conversion devices and inwhich a plurality of pixels, which output electric signals when photonsare incident, are disposed in an array; a sensing circuit section inwhich a plurality of sensing circuits, which receive the electricsignals from the pixels and perform binary determination regardingwhether or not there is an incidence of photons on the pixels in apredetermined period, are arrayed; and a determination resultintegration circuit section having a function of integrating a pluralityof determination results of the sensing circuits for the respectivepixels or for each pixel group. The determination result integrationcircuit section derives the amount of photon incidence on the lightreceiving section by performing photon counting for integrating theplurality of determination results in the plurality of pixels.

According to the embodiments of the present disclosure, it is possibleto enable imaging or light intensity measurement with less noise even atlow illuminance and with a wide dynamic range by making an analog signalunnecessary without reducing the aperture ratio of a pixel.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a view showing an example of the configuration of a CMOS imagesensor (imaging device) according to a first embodiment of the presentdisclosure;

FIG. 2 is a conceptual view showing a light receiving section in thepresent embodiment;

FIG. 3 is a view showing the relationship between the average number oftimes of incidence of photons on a unit grid of a mesh of the lightreceiving section, which is shown in FIG. 2, and the average number ofcounts;

FIG. 4 is a view showing an example of the circuit configuration of apixel in the present embodiment;

FIG. 5 is a view for explaining the cyclic access of pixel blocks in thefirst embodiment;

FIG. 6 is a circuit diagram showing an example of a sensing circuithaving a self-reference function;

FIGS. 7A to 7F are timing charts for explaining an example of a readoperation using the sensing circuit having a self-reference function,which is shown in FIG. 6, in the example of the pixel in FIG. 4;

FIG. 8 is a view for explaining a second embodiment of the presentdisclosure and is also a view showing an example of the configuration ofa pixel block corresponding to the first embodiment using an internalamplification type photodiode;

FIGS. 9A and 9B are conceptual views of an imaging apparatus when theimaging device according to the embodiment of the present disclosure isapplied to CT (Computer Tomography) imaging;

FIG. 10 is a view showing an example of a linear imaging apparatus inwhich the imaging devices (light receiving devices) according to theembodiment of the present disclosure are arrayed in a one-dimensionallinear shape;

FIG. 11 is a view showing the example of radiation-proof protection ofthe imaging device (light receiving device) according to the embodimentof the present disclosure;

FIG. 12 is a schematic view showing an example of estimation of thedirection of radiation incidence by simultaneous detection of photons;

FIG. 13 is a view showing an example of the configuration of a CMOSimage sensor (imaging device) according to a fourth embodiment of thepresent disclosure;

FIG. 14 is a view for explaining the time resolution of photon detectionusing the imaging device according to the fourth embodiment;

FIG. 15 is a view showing an example of the configuration of a CMOSimage sensor (imaging device) according to a fifth embodiment of thepresent disclosure;

FIG. 16 is a view for explaining the time resolution of photon detectionusing the imaging device according to the fifth embodiment; and

FIG. 17 is a view showing an example of the configuration of a camerasystem to which the solid state imaging device according to theembodiment of the present disclosure is applied.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Hereinafter, embodiments of the present disclosure will be describedwith reference to the accompanying drawings. In addition, theexplanation will be given in following order.

1. Outline of features of an imaging device of the present embodiment

2. First embodiment (first exemplary configuration of an imaging device)

3. Second embodiment (second exemplary configuration of an imagingdevice)

4. Third embodiment (application example of an imaging device

5. Fourth embodiment (third exemplary configuration of an imagingdevice)

6. Fifth embodiment (fourth exemplary configuration of an imagingdevice)

7. Sixth embodiment: (camera system)

1. Outline of Features of an Imaging Device of the Present Embodiment

In the present embodiment, an optimal configuration of an imaging device(CMOS image sensor) as a full digital image sensor using photon countingis realized in the field of high-speed parallel reading.

First, each pixel outputs the incidence of a photon within a specificperiod as an electric signal. A sensing circuit receives the resultmultiple times within 1 frame period and performs determination based onthe binary value. The imaging device generates gray-scale data byintegration for each pixel, for example.

The imaging device according to the present embodiment has the followingcharacteristics based on such a basic configuration.

Time-division photon counting makes it unnecessary to continuouslymonitor the generation of a pulse by incidence of a photon in a systemby changing the photon detection process to repetitive detection in afixed cycle.

Focusing on this, in the present embodiment, first, the configuration inwhich each pixel has a separate sensing circuit and a separate counteris not adopted and the three components are hierarchized.

That is, in the present embodiment, a plurality of pixels share onesensing circuit or a plurality of sensing circuits share one countingcircuit given the time-division photon counting.

In the present embodiment, for example, a plurality of pixels whichshare a sensing circuit are read cyclically and exposure is executed fora period from last reading to current reading. Accordingly, theabove-described sharing does not have an adverse influence on theexposure time.

In addition, it is also possible to start the next exposure while countprocessing of determination results and processing for storing the dataon a memory are being executed. Therefore, although time taken for thecount processing increases as a plurality of sensing circuits share acounter, this does not have an adverse influence on the exposure time.

In addition, in the present embodiment, the dynamic range of photoncounting is expanded by adding count results of a plurality of pixels.

Addition between pixels which share a counter can be executed veryeasily by storing the results of different pixels at the same address ofa memory.

In addition, a function of deriving the total amount of incident lightby adding all count results of the light receiving section is set. Forexample, it can be easily realized by providing an additional adder neara data output section.

In addition, by providing such light receiving devices as unit pixelslinearly or in an array, it is possible to detect a very small amount oflight and to perform imaging with a wide dynamic range.

According to the present embodiment adopting the configuration describedabove, it is possible to significantly reduce the circuit size necessaryfor photon counting. Therefore, using the miniaturization technology fora semiconductor imaging device, high-performance photon counting using aplurality of pixels can be executed.

The dynamic range of photon counting is determined by the total numberof meshes using both multi-division in a time direction andmulti-division of the incidence surface, and each mesh has a binaryvalue.

The resolution of meshes and the dynamic range of the number of countsincrease with the development of both miniaturization technology insemiconductor manufacturing and an improvement in the speed.

Although accurate light intensity detection or accurate imaging ispossible with only one imaging device according to the presentembodiment, accurate imaging with a wider dynamic range becomes possibleby arraying the plurality of imaging devices according to the presentembodiment as unit light receiving devices.

Since each light receiving device has a counting function, such a systemcan be built easily without using an expensive external device.

In addition, since each light receiving device performs full digitalcounting which is directly related to the number of incident photonsitself, a sensitivity variation between light receiving devices observedin a typical analog imaging device hardly exists. That is, sensitivityadjustment between light receiving devices is not necessary.

For example, if the imaging device according to the present embodimentis used together with a scintillator for transmission imaging using asmall quantity of X-rays, high-precision and high-resolution imaging canbe executed with the low exposure, and the cost of the system is verylow.

Hereinafter, a CMOS image sensor which is an imaging device according tothe present embodiment having the above characteristics will bedescribed in detail.

2. First Embodiment

FIG. 1 is a view showing an example of the configuration of a CMOS imagesensor (imaging device) according to a first embodiment of the presentdisclosure.

[Outline of the Overall Configuration]

A CMOS image sensor 100 includes a pixel array section 110, a sensingcircuit section 120, an output signal line group 130, a transfer linegroup 140, and a determination result integration circuit section 150.

In the CMOS image sensor 100, a plurality of pixels share one sensingcircuit, as will be described later.

Corresponding to this, the CMOS image sensor 100 includes pixel blocks160-0 to 160-3, . . . , each of which includes a plurality of pixels DPXon the same column, and a selection circuit.

In addition, the CMOS image sensor 100 includes a row control line group180 and a row driving circuit 170 for driving the pixel DPX of the pixelarray section 110 to output an electric signal of the pixel DPX to anoutput signal line 131.

In the pixel array section 110, a plurality of digital pixels DPX arearrayed in a matrix in the row and column directions.

Each digital pixel DPX has a photoelectric conversion device, and has afunction of outputting an electric signal when a photon is incident.

Moreover, as described above, each of the pixel blocks 160-0 to 160-3, .. . is formed by the plurality of pixels DPX on the same column and aselection circuit.

The CMOS image sensor 100 has a circuit block 200 which generatestwo-dimensional imaging data with gradation, for example, by determininga binary value of an electric signal transmitted through the outputsignal line 131 for a fixed period, integrating the determination resultmultiples times for each pixel, and adding the count results of aplurality of pixels.

The CMOS image sensor 100 derives the amount of photon incidence on thepixel array section 110, which is a light receiving section, byintegrating the determination results obtained multiple times for aplurality of pixels, in the present embodiment, for a plurality ofpixels in units of the pixel blocks 160-0 to 160-3, . . . .

The CMOS image sensor 100 has a function of expanding the dynamic rangeof photon counting by adding the count results of a plurality of pixels.

The pixel array section 110, the sensing circuit section 120, and thedetermination result integration circuit section 150 are disposed in thecircuit block 200.

In the sensing circuit section 120, sensing circuits 121-0, 121-1,121-2, 121-3, . . . are arrayed corresponding to the pixel blocks 160-0to 160-3, . . . of the pixel array section 110, respectively.

An input of the sensing circuit 121-0 is connected to an output signalline 131-0 to which outputs of all pixels DPX-00, DPX-10, . . . , andDPX-p0, which form the pixel block 160-0, are connected in common.

That is, the plurality of pixels DPX-00 to DPX-p0 shares the one sensingcircuit 121-0.

In addition, the number of pixels in each pixel block 160 (160-0 to160-3) is set to 128, for example. In this case, p is 0 to 127, and thepixel block 160-0 includes pixels DPX-00 to DPX1270.

An input of the sensing circuit 121-1 is connected to an output signalline 131-1 to which outputs of all pixels DPX-01, DPX-11, . . . , andDPX-p1, which form the pixel block 160-1, are connected in common.

That is, the plurality of pixels DPX-01 to DPX-p1 shares the one sensingcircuit 121-1.

The pixel block 160-1 includes 128 pixels DPX-01 to DPX1271, forexample.

An input of the sensing circuit 121-2 is connected to an output signalline 131-2 to which outputs of all pixels DPX-02, DPX-12, . . . ,DPX-p2, which form the pixel block 160-2, are connected in common.

That is, the plurality of pixels DPX-02 to DPX-p2 share the one sensingcircuit 121-2.

The pixel block 160-2 includes 128 pixels DPX-02 to DPX1272, forexample.

An input of the sensing circuit 121-3 is connected to an output signalline 131-3 to which outputs of all pixels DPX-03, DPX-13, . . . ,DPX-p3, which form the pixel block 160-3, are connected in common.

That is, the plurality of pixels DPX-03 to DPX-p3 share the one sensingcircuit 121-3.

The pixel block 160-3 includes 128 pixels DPX-03 to DPX1273, forexample.

In the sensing circuit section 120, also for other pixel blocks (notshown), sensing circuits are arrayed so as to be shared by a pluralityof pixels.

The determination result integration circuit section 150 has a functionof generating two-dimensional imaging data with gradation, for example,by integrating determination results of the sensing circuits 121-0 to121-3 multiples times for each pixel and adding the count results of theplurality of pixels.

The determination result integration circuit section 150 has a functionof deriving the amount of photon incidence on the pixel array section110, which is a light receiving section, by integrating thedetermination results obtained multiple times for a plurality of pixels,in the present embodiment, for a plurality of pixels in units of thepixel blocks 160-0 to 160-3, . . . .

The determination result integration circuit section 150 has registers151-0 to 151-3, a selection circuit 152, a counting circuit 153, and amemory 154.

The registers 151-0 to 151-3 holds determination values of thecorresponding sensing circuits 121-0 to 121-3 transmitted throughtransfer lines 141-0 to 141-3.

The selection circuit 152 selects outputs of the registers 151-0 to151-3 sequentially to supply the determination values, which are held inthe respective registers 151-0 to 151-3, to the counting circuit 153.

The counting circuit 153 performs count processing on the determinationvalues of a plurality of pixels (4 pixels in this example), which areselected by the selection circuit 152 after being read by row selection,and stores the count result for each pixel in the memory 154.

Then, the counting circuit 153 adds the count results of a plurality ofpixels and stores the addition result in the memory 154.

Pixel data at the time of last reading is loaded from the memory 154 tothe counting circuit 153.

In the first embodiment, the determination result integration circuitsection 150 includes one counting circuit 153, and the plurality ofregisters 151-0 to 151-3 share the counting circuit 153.

In other words, the CMOS image sensor 100 according to the firstembodiment shares the counting circuit 153 among the plurality ofsensing circuits 121-0 to 121-3.

The CMOS image sensor 100 according to the present embodiment isconfigured to have the above-described characteristic configuration.

That is, the CMOS image sensor 100 is configured to share a sensingcircuit between a plurality of pixels for cyclic access, so thatexposure time can be secured and it can meet a small pixel.

In addition, since a plurality of sensing circuits share a countingcircuit, it is possible to form the CMOS image sensor 100 with optimalcircuit size and processing speed.

The CMOS image sensor 100 has a function of expanding the dynamic rangeof photon counting by adding the count results of a plurality of pixels.

Here, the basic concepts of light reception and photon counting of alight receiving section 300, which is formed by the pixel array section110 in the circuit block 200 of the CMOS image sensor 100 according tothe present embodiment, will be described with reference to FIGS. 2 and3.

FIG. 2 is a conceptual view showing the light receiving section 300 inthe present embodiment.

FIG. 3 is a view showing the relationship between the average number oftimes of incidence of photons on a unit grid of a mesh of the lightreceiving section, which is shown in FIG. 2, and the average number ofcounts.

In addition, in FIG. 2, the originally two-dimensional light receivingsurface is expressed in a one-dimension manner for the sake ofsimplicity.

The photon counting is executed by forming three-dimensional meshes MSHin the light receiving section 300 using a light receiving surface 310divided at equal distances and a time axis t divided at equal distances(expressed in a two-dimensional manner in FIG. 2).

Each mesh MSH has a binary value. That is, the sensing circuit section120 determines whether or not one or more photons are incident on eachmesh MSH. In this case, for example, “1” is determined regardless of thenumber of incident photons if there is an incidence and “0” isdetermined if there is no incidence. In FIG. 2, a mesh blockcorresponding to “1” is displayed with a thick frame. In addition, thereference numeral IVT in FIG. 2 indicates an incidence event of aphoton.

In addition, the total number of “1” is counted by the determinationresult integration circuit section 150 and is then stored in the memory154.

Here, assuming that photons are incident appropriately uniformly withrespect to the time axis t while fluctuating and are also incidentappropriately uniformly in the surface direction, the relationshipbetween the total number of counts and the actual number of incidentphotons follows the Poisson distribution.

FIG. 3 is a view showing the relationship between the average number oftimes of incidence of photons on a unit grid CL of a mesh and theaverage number of counts.

As shown in FIG. 3, the number of times of incidence is substantiallyequal to the number of counts in a fine light region where the averagenumber of times of incidence is 0.1 times or less.

In addition, if the average number of times of incidence is 0.4 times orless, the relationship between the number of times of incidence and thenumber of counts is approximately linear.

That is, if the total number of grids of the mesh MSH is sufficientlylarger than the number of incident photons, the count value reflects thenumber of incident photons linearly, and highly precise counting ispossible accordingly.

In addition, it is possible to improve the accuracy of counting whileexpanding the dynamic range by narrowing the mesh spacing in the surfacedirection or on the time axis t to increase the total number of grids.

That is, using high-speed circuit technology and miniaturizationtechnology in semiconductor manufacturing, it is possible to improve theaccuracy of photon measurement and expand the dynamic rangesignificantly in the light receiving section 300.

In addition, the following configuration is effective when the incidenceof light in the surface direction is largely biased locally and theamount of incident light is relatively large.

The measurement accuracy can be improved by dividing a surface directionmesh into a plurality of groups formed by one or more grid blocks,calculating the average number of counts of the grids CL for each group,and performing correction according to the Poisson distribution.

Alternatively, it is also effective to ease the deviation of incidentphotons in the surface direction by disposing an optical low pass filterbefore the light receiving surface 310. Moreover, in the case of X-raydetection using a scintillator, the scintillator itself serves as anoptical low pass filter since light is emitted from the scintillatorwhile scattering when an X-ray is incident.

[Function Related to a Digital Pixel]

Here, an example of the configuration of the digital pixel DPX will bedescribed.

As described above, the digital pixel (hereinafter, simply referred toas a pixel) DPX has a photoelectric conversion device and outputs anelectric signal when a photon is incident.

Since the CMOS image sensor 100 as an imaging device has a resetfunction and a read function for the pixel DPX, it is possible toexecute resetting and reading at arbitrary timing.

The resetting refers to resetting the pixel DPX to the state where aphoton is not incident. Preferably, each pixel DPX includes a lens onthe light receiving surface, or may further include a color filter onthe light receiving surface when necessary.

Although such a basic function of a pixel is close to that of a normalpixel, the accuracy or linearity as an analog value is not necessary forthe output.

Here, an example of the configuration of a digital pixel will bedescribed.

FIG. 4 is a view showing an example of the circuit configuration of apixel in the present embodiment.

FIG. 4 shows an example of a pixel circuit including three transistorsin one unit pixel DPX.

The one unit pixel DPX includes a photodiode 111, a transfer transistor112, a reset transistor 113, an amplifier transistor 114, anaccumulation node 115, and a floating diffusion (FD) node 116.

A gate electrode of the transfer transistor 112 is connected to atransfer line 181 serving as a row control line, and a gate electrode ofthe reset transistor 113 is connected to a reset line 182 serving as arow control line.

A gate electrode of the amplifier transistor 114 is connected to the FDnode 116, and a source electrode of the amplifier transistor 114 isconnected to the output signal line 131.

In the pixel DPX, a pair of electron and hole are generated by lightincident on a silicon substrate of the pixel, and electrons areaccumulated at the accumulation node 115 by the photodiode 111.

These are transmitted to the FD node 116 by turning on the transfertransistor 112 at predetermined timing, thereby driving the gate of theamplifier transistor 114.

As a result, a signal charge is read as a signal to the output signalline 131.

The output signal line 131 may be grounded through a constant currentsource or a resistive device for a source-follower operation, or may begrounded before reading and then have a floating state so that thecharge level based on the amplifier transistor 114 is output.

The reset transistor 113 resets a pixel to the dark state beforeaccumulation, that is, to the state where a photon is not incident byextracting electrons accumulated in the photodiode 111 when turned onsimultaneously with the transfer transistor 112.

Such a circuit or an operation mechanism of a pixel is the same as thatof an analog pixel, and various kinds of variations may occur similar tothe analog pixel.

However, a digital pixel outputs the incidence of one photon in adigital manner while an analog pixel outputs the total incidence amountof a plurality of photons in an analog manner.

Accordingly, the digital pixel and the analog pixel are differentlydesigned.

First, in the case of a digital pixel, it is necessary to generate asufficiently large electric signal for the incidence of one photon.

For example, in the pixel circuit with an amplifier transistor shown inFIG. 4, it is preferable that the parasitic capacitance of the FD node116, which is an input node of the amplifier transistor 114 which formsa source follower, be set as small as possible.

Also in this case, it is preferable that the amplitude of an outputsignal with respect to the incidence of one photon be kept sufficientlylarger than that of random noise of the amplifier transistor 114.

On the other hand, since it is not necessary to consider the linearity,the accuracy, or the operation range in an output signal from a pixelunlike an analog pixel, the same low voltage as for the digital circuitcan be used for an I/O power source of a source follower, for example.Moreover, the accumulated charge capacity of a photodiode is preferablyset to be as small as possible.

Next, the outline of the overall operation of the CMOS image sensor 100according to the first embodiment will be described.

As described above, the pixel block 160 (160-0 to 160-3, . . . )includes 128 digital pixels DPX and a selection circuit. The selectioncircuit selects one of the pixels to execute resetting or reading.

In this example, one pixel in the pixel block 160 is selected accordingto the row control lines 181 and 182 driven by the row driving circuit170.

At the time of reading, whether or not there is an incidence of a photonon the selected pixel is output as an electric signal to the outputsignal line 131 (131-0 to 131-3, . . . ) and the binary value isdetermined by the sensing circuit 121 (121-0 to 121-3).

For example, the sensing circuit 121 (121-0 to 121-3) determines “1” asa determination value if light is incident on the selected pixel anddetermines “0” as a determination value if light is not incident on theselected pixel and latches the determination value.

The determination value of the sensing circuit 121 (121-0 to 121-3) isfirst transmitted to the register 151 (151-0 to 151-3).

The counting circuit 153 is shared by the four pixel blocks 160-0 to160-3, and count processing on four pixels read by row selection isexecuted sequentially through the selection circuit 152.

In addition, the count result for each pixel is stored in the memory154.

That is, pixel data at the time of last reading is first loaded from thememory 154 to the counting circuit 153.

In this case, the counting circuit 153 adds “1” to the count value if“1” is stored in the register 151 (151-0 to 151-3) and does not updatethe count value if “0” is stored.

Then, the value of the counting circuit 153 is returned to the memory154, and the count processing for one pixel is completed. Thisprocessing is executed sequentially for four pixels.

While such count processing is being executed, the pixel block 160(160-0 to 160-3) and the sensing circuit 121 (121-0 to 121-3) canexecute reading and determination of the next row in parallel.

For example, such digital reading is executed 1023 times in one frameperiod to form 10-bit gray-scale data for each pixel.

In this case, the counting circuit 153 is 10 bits, and the memory 154 is5120 bits since each of “128×4” pixels has 10-bit data.

That is, the CMOS image sensor 100 operates as a photon counter arrayedwith a unique configuration.

Incidentally, the size of the counting circuit 153 or the memory 154changes with applications.

For example, when forming an imaging unit with “4 pixels wide×4 pixelslong”, data of pixels included in each imaging unit is stored in thesame address of the memory 154.

Then, the count value of the incidence of photons on the 16 pixels isadded in the counting circuit 153 through the memory.

In this case, the total number of counts becomes 16 times, and 14 bitsis necessary for the counting circuit 153.

On the other hand, the number of addresses in the memory 154 is set to32/( 1/16), and each stores a 14-bit value. Accordingly, the necessarycapacity is 448 bits.

Alternatively, where counting only the total number of photon incidenceson the entire light receiving surface, it is not necessarily to providea memory since the data is held in the counting circuit 153.

In this case, 19 bits corresponding to 10-bit counts for 512 pixels arenecessary for the number of bits in a counter.

Alternatively, when changing a function from two-dimensional imaging ofall pixels to totaling according to applications, 14 bits are set forthe counting circuit 153 and the 14-bit memory 154 is prepared for“128×4” pixels. In addition, the level of the circuit block 200 is setto meet “4×4” addition.

For addition of all pixels, it is preferable to execute the “4×4”addition by the circuit block 200 first, to prepare a separate adder inan output circuit, and to calculate the total by adding a plurality ofoutput values from the memory 154. In this case, since the throughput ofan adder of an output unit is 1/16 of that in the case where there is noprior addition, high-speed processing is not necessary.

Next, cyclic access of pixel blocks in the first embodiment will bedescribed.

FIG. 5 is a view for explaining the cyclic access of pixel blocks in thefirst embodiment.

Here, an example where a pixel block is formed by 16 pixels and onesensing circuit is shared is shown for the sake of simplicity.

16 pixels included in each pixel block 160 (160-0 to 160-3, . . . ) aresequentially accessed in a cyclic manner.

Assuming that the frame rate is 1/30 second and reading of each pixel isexecuted 1023 times during this time, 1 cycle of block processing isabout 32 microseconds. It is necessary to complete reading of 16 pixelsduring this time.

Time division on the vertical axis in FIG. 5 is a time t assigned toaccess to each pixel in a block, and the maximum width is 2microseconds.

In addition, when the pixel block 160 (160-0 to 160-3, . . . ) includes128 pixels as in the example shown in FIG. 1, access time of each pixelis 250 nanoseconds.

Since data reading from each pixel and data determination are a simpleoperation similar to reading of a semiconductor memory, there aresufficient allowances in this time width.

In the cyclic access described above, resetting RST and reading RD ofeach pixel DPX are cyclically executed.

In this case, access timing differs according to each pixel, but a timefor substantial exposure EXP from the resetting RST to the reading RD isequal for all pixels.

Since the exposure time can be changed by changing the timing of theresetting RST within a range of a cycle, it is possible to adjust thesensitivity without having an effect on other circuit operations.

For example, if the resetting RST is set immediately after the lastreading RD (the same time division as for reading) in each pixel DPX,the exposure time becomes a maximum and this corresponds tolow-illuminance subject imaging.

On the contrary, if the resetting RST is set immediately before thereading RD (time division before one-time reading), the exposure timebecomes shortest and this corresponds to high-illuminance subjectimaging. In addition, if the reset timing is changed through severalsteps in the same time division, the exposure time can be selected morefreely.

Although count processing CNT is executed successively after the readingRD, but reading of the next pixel is started in parallel.

Here, for example, pixel No. 4 is read at time t4, and pixel No. 1 isreset. Moreover, in parallel with this, count processing of pixel No. 3is executed.

In this example, the reading of the pixel No. 4 and the resetting of thepixel No. 1 are executed in series in a time-division manner. In thecase of a pixel shown in FIG. 4 which has a separate reset mechanism ineach pixel, however, the reading of the pixel No. 4 and the resetting ofthe pixel No. 1 may be executed simultaneously and in parallel bydriving two row control lines.

As described above, the CMOS image sensor 100 according to the firstembodiment has a hierarchical structure in which the plurality of pixelsDPX share the sensing circuit 121 (121-0 to 121-3) and the register 151(151-0 to 151-3) and the plurality of sensing circuits 121 (121-0 to121-3) share the counting circuit 153.

Each sharing rate can be optimized on the basis of the relationshipbetween the access time and the occupying area of each circuit.

In addition, the circuit block 200 shown in FIG. 1 which has four pixelblocks may be arrayed in a plural number in the horizontal direction(column arrangement direction).

For example, a light receiving device including “128×128” pixels can beformed by arraying 32 circuit blocks 200 in parallel and making themoperate in parallel. The performance of such a light receiving device isestimated below.

It is assumed that imaging of 10 bits of each pixel is executed with 30frames per second.

When the numbers of counts of all pixels are added and the result isused in a single photon counter, the maximum value of the total numberof counts of photons per second is calculated as “128×128×1023×30”,reaching 500 million.

Even if only a linear region of the Poisson distribution is used, themaximum value is 200 million. If correction is made, counting beyond theabove is also possible.

Moreover, as described above, such a light receiving device may be usedfor two-dimensional imaging according to its application, and may alsobe used as a single light receiving device for photon counting.

They can change the operation mode easily by rewriting the internalregister value from the outside. Changing the exposure time by changingthe reset timing is also programmable in the same method.

Moreover, as described above, a digital pixel used in the presentembodiment has a photoelectric conversion device and has a function ofoutputting an electric signal according to the incidence of a photon.For example, the digital pixel used in the present embodiment isconfigured as shown in FIG. 4.

Moreover, in order to offset the output variations in pixels whenreading the data read from digital pixels, it is desirable to introducethe following self-reference function at the time of sensing.

That is, an output in a reset state and a signal output after exposureare read from a pixel, and a sensing circuit adds an offset value toeither one of them and compares a signal, which is obtained by additionof the offset value, with the other signal to execute binarydetermination.

FIG. 6 is a circuit diagram showing an example of a sensing circuithaving a self-reference function.

A sensing circuit 121A shown in FIG. 6 includes switches SW121, SW122,and SW123, capacitors C121 and C122, inverters IV121 and IV122, and asupply line L121 of an offset signal OFFSET.

A terminal a of the switch SW121 is connected to a first terminal of thecapacitor C121 and a first terminal of the capacitor C122, and aterminal b of the switch SW121 is connected to a terminal SIG connectedto an output signal line.

A second terminal of the capacitor C121 is connected to an inputterminal of the inverter IV121, a terminal a of the switch SW122, and aterminal a of the switch SW123.

A second terminal of the capacitor C122 is connected to the supply lineL121 of the offset signal OFFSET.

An output terminal of the inverter IV121 is connected to an inputterminal of the inverter IV122 and a terminal b of the switch SW122.

An output terminal of the inverter IV122 is connected to a terminal b ofthe switch SW123 and an output terminal SAOUT.

Here, an example of a read operation using the sensing circuit having aself-reference function, which is shown in FIG. 6, in the example of thepixel in FIG. 4 will be described.

FIGS. 7A to 7F are timing charts for explaining an example of the readoperation using the sensing circuit having a self-reference function,which is shown in FIG. 6, in the example of the pixel in FIG. 4.

FIG. 7A shows a reset pulse RESET applied to the reset line 182 in FIG.4, and FIG. 7B shows a read pulse READ applied to the transfer line 181in FIG. 4.

FIG. 7C shows an ON/OFF state of the switch SW121, FIG. 7D shows anON/OFF state of the switch SW122, FIG. 7E shows an ON/OFF state of theswitch SW123, and FIG. 7F shows the offset signal OFFSET.

First, the switches SW121 and SW122 are turned ON to apply the resetpulse RESET to the reset line 182 of the pixel DPX, so that a pixeloutput in the reset state is read to the input terminal SIG.

Then, the switch SW122 is turned OFF to hold the reset output.

Then, the pulse READ is applied to the transfer line 181 of the pixelDPX and the signal output, which is an exposure result, is input to theterminal SIG to turn OFF the switch SW121.

During this period, the offset signal OFFSET input is kept at 0 V.

Then, the level of the offset signal OFFSET is slightly increased to addan offset electric potential to the read signal through the capacitorC122.

Then, the output in the reset state is compared with the output obtainedby adding a small offset value to the read signal.

When a photon is incident on the pixel shown in FIG. 4, the lattersignal has a lower electric potential than the former signal.Accordingly, “0” is output to the output terminal SAOUT.

When a photon is not incident on the pixel, “1” is output to the outputterminal SAOUT in the opposite manner as when a photon is incident onthe pixel.

Finally, the switch SW123 is turned ON to latch the determinationresult.

Such a self-reference function offsets fixed noise in each pixel, whichoccurs due to a variation in the threshold value of the amplifiertransistor 114, so that it also becomes possible to perform accuratebinary determination for a small signal. In addition, in theabove-described sequence, kTC noise occurring at the time of resettingis also offset.

In addition, a similar effect can also be expected in correlated doublesampling (CDS) at the time of AD conversion of an analog signal.

In this case, time taken for double reading and determination is fixedall the time in sensing for binary determination. Accordingly, theinfluence of thermal noise or flicker noise from an amplifier transistorof a pixel or the sensing circuit itself can also be reduced as follows.

That is, since noise in a low frequency band is often superimposedsimilarly in both reading, the influence can be offset. In addition,noise in a high frequency band may limit the sensitivity of a capacitiveload of a sensing circuit.

Accordingly, by setting the capacitive load as large as possible withinthe range in which sensing can be correctly performed, it is possible tonarrow the bandwidth of influential noise to the minimum.

In the correlated double sampling at the time of AD conversion, timetaken for conversion changes in many cases according to the signalintensity or the number of bits. For this reason, the circuit isnecessarily influenced by the noise in a wide band.

Without being limited to the example described above, the circuit mayexecute determination by comparing a signal, which is obtained by addingan offset value to a reset signal, with a read signal.

Alternatively, it is also possible to acquire a read signal first andthen to reset a pixel to acquire a reset signal and to add an offsetvalue to either one of the signals for comparison determination. In thiscase, it is difficult to offset the kTC noise, but the fixed noisecaused by variations in pixels can be offset. Accordingly, there is anadvantage in that this can be generally applied to all pixelconfigurations.

Even if such a self-reference function is applied, the number of sensingcircuits is much smaller than that in a normal AD converter.Accordingly, a large occupying area is not necessary.

Moreover, in order to realize a digital pixel, it is also a good choiceto use an internal amplification type photodiode.

As the internal amplification type photodiode, an avalanche photodiode(APD) is known which generates avalanche amplification by acceleratingphotoelectrically converted electron-hole pairs by an electric field.

A photon counter in the related art which uses the APD performs onlyanalog amplification of a pixel signal, performs the pulse output, anddetects it by an external circuit. In this case, amplification near 1million times is executed to detect a single photon using the Geigermode. Accordingly, a high voltage of 40 V is necessary, and a detectioncircuit is not provided either. For this reason, miniaturization of apixel or a high-speed parallel operation is difficult.

On the other hand, the Geiger mode operation is not necessary for thedigital pixel applied to the present embodiment. Since time-divisionbinary detection in a chip using a simple circuit configuration cansignificantly reduce the detection circuit noise and the signal load, itis possible to detect a single photon with a small gain in a linearmode.

Also in this case, the pixel circuit shown in FIG. 4 can be used, but anamplifier transistor of a pixel is not necessary if amplification of1000 times is obtained, for example.

3. Second Embodiment

Next, an example of the configuration in which an internal amplificationtype diode is applied to a light receiving device will be described as asecond embodiment.

FIG. 8 is a view for explaining the second embodiment of the presentdisclosure and is also a view showing an example of the configuration ofa pixel block corresponding to the first embodiment using an internalamplification type photodiode.

In the second embodiment, a pixel block 160B includes only a group ofinternal amplification type photodiodes 111B and transfer (selection)transistors 112B corresponding to the internal amplification typephotodiodes 111B.

That is, the pixel DPXB in this example is formed only by the internalamplification type photodiodes 111B and the transfer (selection)transistors 112B corresponding to the internal amplification typephotodiodes 111B. A gate electrode of the transfer transistor 112B ofeach pixel DPXB on the same row is connected to a common transfer line181B. In addition, sources or drains of the transfer transistors 112B ofa plurality of pixels of each pixel block 160B are connected to thecommon output signal line 131.

In addition, a reset transistor 113B is connected between each outputsignal line 131 and a reset potential line LVRST. A gate electrode ofeach reset transistor 113B is connected to the common reset line 182B.

In this example, each pixel DPXC is reset through the reset transistor113B, the output signal line 131, and the transfer transistor 112B.

4. Third Embodiment

Next, an example of the configuration of an imaging apparatus using aplurality of light receiving devices (light receiving units and circuitblocks) of the imaging device according to the first or secondembodiment will be described as a third embodiment.

In a semiconductor imaging apparatus represented by a general CCD typeor CMOS sensor type imaging device, there are characteristic variationsin amplifier circuits of a CCD output unit or in source-followercircuits connected to respective pixels of a CMOS sensor.

Moreover, in a general semiconductor imaging apparatus, thischaracteristic variation is reflected as it is on a variation in theefficiency of conversion from the number of accumulated electrons intoan analog electric signal.

In addition, since conversion variations in AD converters are alsodirectly reflected on signal variations, variations in the effectivesensitivity of respective chips are very large.

Therefore, when performing large-area imaging by arraying the pluralityof imaging devices in a common semiconductor imaging apparatus, it isnecessary to make the sensitivity uniform by adjusting the gain of eachchip.

On the other hand, since the imaging device (light receiving device)according to the embodiment of the present disclosure to whichtime-division photon counting is applied does not treat an analog signalbasically, sensitivity variations in respective chips are very small.

Accordingly, it is possible to form a large imaging surface by arrayingthese imaging devices in a one-dimensional linear shape or in atwo-dimensional array.

For example, such an imaging apparatus can be used for radiation imagingfor medical or security applications by disposing a scintillator infront of a light receiving device. In addition, since the sensitivity ishigh and amount of noise is small, the imaging apparatus can detect avery small amount of radiation correctly.

Accordingly, for example, in medical imaging, it is possible tosignificantly reduce the amount of exposure to an object to be imaged bylimiting the amount of radiation.

FIGS. 9A and 9B are conceptual views of an imaging apparatus when theimaging device according to the embodiment of the present disclosure isapplied to CT (Computer Tomography) imaging.

An imaging apparatus 400 which surrounds a subject OBJ in a cylindricalshape includes an X-ray source 410 and thousands of imaging devices 420,which are disposed in an array so as to face the X-ray source 410 andwhich use photon counters according to the embodiment of the presentdisclosure.

The array surface is curved along the inner wall of the cylinder so thatthe imaging devices face the X-ray source 410 at equal distancestherebetween.

In each imaging device 420, a scintillator 422 is attached to a lightreceiving surface 421 a side of a photon counter 421 in the embodimentof the present disclosure, and a collimator 423 is disposed at theincidence side of X-rays.

X-rays which are transmitted through the subject OBJ from the X-raysource 410 and are then transmitted through the collimator 423 areconverted into visible light by the scintillator 422 and are detected bythe photon counter 421, and the amount of radiation is derived.

The imaging apparatus 400 rotates around the subject OBJ to image thesubject OBJ at all angles, and arithmetic processing on the obtaineddata is executed to generate a cross-sectional transmission image of thesubject OBJ.

The photon counter of the imaging device according to the embodiment ofthe present disclosure has a very high dynamic range as well ashigh-sensitivity reading and no noise.

In addition, since the imaging device includes a counting circuitinside, it is possible to perform high-speed imaging even at high-bitresolution. Accordingly, accurate imaging can be realized even if thequantity of X-rays is significantly reduced, and the system is notexpensive.

As an example of a similar imaging system, there is a SPECT for medicalapplications.

This detects γ-rays using a scintillator, but a photomultiplier tube isused to detect a very small quantity of γ-rays.

If the photon counter in the embodiment of the present disclosure isused, the cost of a detector is significantly reduced, and an externaldetection circuit is not necessary either. As a result, since the numberof detectors can be increased tens of times, it is possible to improvethe sensitivity significantly.

FIG. 10 is a view showing an example of a linear imaging apparatus inwhich the imaging devices (light receiving devices) according to theembodiment of the present disclosure are arrayed in a one-dimensionallinear shape.

Imaging devices (light receiving devices) 510 according to theembodiment of the present disclosure are arrayed linearly andalternately in a linear imaging device 500.

By moving the linear imaging device 500 in a direction of arrow A, awide imaging surface can be uniformly scanned in an effective pixelregion 520 of the imaging device (light receiving device) 510.

For the scanning, it is preferable to make stepwise movement at pitchesin the longitudinal direction (row direction) of the effective pixelregion 520, or it is possible to move a subject. A connection partbetween effective pixel regions may be subjected to averaging processingby making some pixels overlap each other.

The effective pixel region 520 of each imaging device (light receivingdevice) 510 has a configuration in which 128 blocks of the pixel arraysection shown in FIG. 1 are arrayed in the horizontal direction (columndirection), for example.

That is, the effective pixel region 520 of each imaging device (lightreceiving device) 510 is formed by “512×128” physical pixels.

Here, assuming that an addition result of count values of “8×8” physicalpixels is a pixel unit (logic pixel), the number of logic pixels is64×16. When each physical pixel has a resolution of 10 bits, theresolution of each logic pixel is 16 bits.

If such 64 imaging devices (light receiving devices) 510 are linearlyarrayed as shown in FIG. 10, total 4096 16-bit logic pixels are arrayedin the linear imaging device 500.

Such a linear imaging apparatus can realize miniature imaging easily.Accordingly, X-ray imaging for medical or security applications becomespossible with high precision and very high sensitivity (low noise) bycombination with a scintillator.

Since the absolute quantity of X-rays can be reduced, it is possible tosuppress the amount of exposure even in the case of line imaging. Inaddition, the system is not expensive. In addition, a plurality of suchlinear imaging apparatuses may be arrayed at equal distances in thescanning direction in order to shorten the scanning distance. In thiscase, the amount of exposure can be further reduced.

Moreover, in order to prevent an X-ray transmitted through thescintillator from damaging the imaging device, it is possible to placethe imaging device 420 at a location distant from the scintillator 422and transmit the emission of the scintillator to the imaging device 420using an optical fiber 424 as shown in FIG. 11, for example.

In the example shown in FIG. 11, an X-ray shield plate 425 which blocksX-rays is disposed between the light receiving surface 421 a of thephoton counter 421 of the imaging device 420 and a light receiving unitof the scintillator 422, and the optical fiber 424 is disposed so as tobypass the X-ray shield plate 425.

On the other hand, for radiation detection in measurements in themedical or scientific fields, the irradiation angle of radiation may benecessary as information. For a photon counter used in such a case, hightime resolution for specifying the detection time is necessary.

For example, in a PET used for medical applications, a positron isgenerated by a radioactive material administered to a patient, and thepositron is combined with an electron immediately to excite a pair ofγ-rays. The pair of γ-rays are emitted in opposite directions and aredetected simultaneously and in parallel by two detectors(scintillators). Thus, the existence of a radioactive material isestimated on the straight line connecting two detectors.

Generally, in the PET, it is necessary to reduce noise at the time ofdetection by executing the determination of simultaneous detection withhigh time resolution.

FIG. 12 is a schematic view showing an example of estimation of thedirection of radiation incidence by simultaneous detection of photons.

FIG. 12 shows a simple application in the SPECT.

By γ-rays incident perpendicular to the scintillator (detector) 422among γ-rays emitted from the subject OBJ, many photon groups areincident simultaneously on the photon counter 421 of one imaging device420.

On the other hand, by γ-rays which are obliquely incident on thescintillator (detector) 422, photon groups distributed in the pluralityof imaging devices 420 are incident simultaneously.

Thus, it is possible to estimate the incidence direction of γ-rays usingthe information regarding the distribution of photons simultaneouslydetected.

Usually, a collimator is used in the SPECT to use the informationregarding only a photon which is vertically incident. However, if thetime resolution of detectors is high and they can be easily used, it ispossible to expand the amount of information more significantly.

That is, in order to improve the detection accuracy by reducing adetection error in such a detector, high time resolution for determiningsimultaneous detection of the photon incidence is important.

Hereinafter, a new technique for improving the time resolution ofoptical detection in the photon counter related to the embodiment of thepresent disclosure and the chip architecture will be described as thirdand fourth embodiments.

5. Fourth Embodiment

FIG. 13 is a view showing an example of the configuration of a CMOSimage sensor (imaging device) according to a fourth embodiment of thepresent disclosure.

A CMOS image sensor (imaging device) 100C related to the fourthembodiment is different from the CMOS image sensor 100 according to thefirst embodiment shown in FIG. 1 in that it has a function of improvingthe time resolution of optical detection.

Basically, the CMOS image sensor 100C is configured such that adetermination result integration circuit section 150C has a function ofimproving the time resolution of optical detection.

The determination result integration circuit section 150C includes asensing circuit section 120, first and second register sections 210 and220, a 4-bit bus 230, and an output circuit 240.

The first register section 210 has 4-bit registers 211-0, 211-1, . . .which transfer outputs of sensing circuits 121-0, 120-1, . . .sequentially corresponding to the column arrangement of pixels of thepixel array section 110.

The first register section 210 has a configuration equivalent to theconfiguration in which line buffers, which hold and output the read dataof one row, are arrayed in four rows.

The second register section 220 has 4-bit registers 221-0, 221-1, . . .which transfer outputs of the 4-bit registers 211-0, 211-1, . . . of thefirst register section 210 sequentially.

The second register section 220 has a configuration equivalent to theconfiguration in which line buffers, which hold and output the read dataof one row, are arrayed in four rows.

The bus 230 transmits the output data of the second register section 220to the output circuit 240.

The output circuit 240 has a counting circuit 241 and an output latch242. The counting circuit 241 counts or adds the data of “1” of each rowtransmitted through the bus 230.

Also in the fourth embodiment, the pixel block 160 (160-0, 160-1, . . .) is configured to include 128 digital pixels DPX and a selectioncircuit as in the first embodiment. The selection circuit selects one ofthe pixels to execute resetting or reading.

Also in the fourth embodiment, one pixel in the pixel block is selectedaccording to a row control line 180 driven by the row driving circuit170.

At the time of reading, whether or not there is an incidence of a photonon the selected pixel is output as an electric signal to the outputsignal line 131 and the binary value is determined by the sensingcircuit 121 (121-0, 121-1, . . . ).

For example, the sensing circuit 121 (121-0, 121-1, determines “1” as adetermination value if light is incident on the selected pixel anddetermines “0” as a determination value if light is not incident on theselected pixel and latches the determination value.

Then, the determination value of the sensing circuit 121 (121-0, 121-1,. . . ) is transmitted to the first bit of the 4-bit register 211(211-0, 211-1, . . . ) of the first 4-bit register section. Accordingly,signal reading and determination of the next row become possible.

Such an operation is continuously performed for four rows. When thedetermination values of the respective rows are stored in different bitsof the 4-bit registers 211 (211-0, 211-1, . . . ), they aresimultaneously transferred to the 4-bit registers 221 (221-0, 221-1, . .. ) of the second register section 220 at the next stage.

Then, the data held in the 4-bit registers 220 (220-0, 220-1, . . . ) ofthe second register section 220 in each column is sequentially output tothe 4-bit bus 230 and is then transmitted to the output circuit 240.

The counting circuit 241 is disposed in the output circuit 240 in orderto count or add the data of “1” of each row. After all column data itemsof four rows are transferred, the addition value of each row is storedin the output latch 242.

On the other hand, reading of the pixel array section 110 iscontinuously executed in parallel with the transfer operation describedabove, and determination values of the next four rows are stored in the4-bit registers 211 (211-0, 211-1, . . . ) of the first register section210. That is, reading and transfer of data to the output circuit 240 arepipelined.

In such a chip, assuming that it takes 250 nanoseconds to performreading of one row, data transfer of 128 columns is performed for 1microsecond.

Since 4-bit transfer of one column is 7.8 nanoseconds, time for datatransfer in a normal semiconductor circuit is sufficient. The peripheralcircuit configuration is very simple.

In addition, for data reading from the outside, it is preferable toacquire the count values of four rows stored in the output latch 242 ofthe output circuit 240 for 1 microsecond.

Since this is a very sufficient time for the reading, an external systemcan read the data in parallel from many imaging devices.

The external system can derive the total number of photons, which areincident on imaging devices within the unit exposure time of maximum 32microseconds, by adding the read data of all rows.

By repeating this 1025 cycles and adding the count values continuously,it is possible to obtain 24-bit gray-scale data for 1/30 second.

Here, time resolution of photon detection using the imaging deviceaccording to the fourth embodiment will be described with reference toFIG. 14.

FIG. 14 is a view for explaining the time resolution of photon detectionusing the imaging device according to the fourth embodiment. FIG. 14shows a state in which reading and resetting are sequentially executedfor each row according to the elapse of time.

In a PET or the like, when γ-rays are incident on a scintillator, manyphotons are generated to be incident on corresponding imaging devices.

Assuming that this timing is a dotted line 251, a photon is selectivelydetected only in row reading (expressed with thick oblique lines: RD) inwhich exposure time includes this time.

In this example, detection is performed until the row address takesnearly one round from reading immediately after the generation of aphoton (row address: 7), and then the data becomes zero. That is, if anoutput of row data of 1 or more occurs continuously or intermittentlyduring a period for which the row address takes a round, this is photongeneration.

Here, the total number of photons which are incident simultaneously onimaging devices is the total addition value of row data outputs for oneround. In addition, it can be estimated that the generation time is at252 between the read time of a row, in which an output of 1 or moreappears first, and the read time of a row before the row in which anoutput of 1 or more appears first. The time resolution is a read timefor one row, that is, 250 nanoseconds.

That is, using this method, the incidence time when a plurality ofphotons are incident simultaneously on imaging devices is specified fromthe distribution of the number of incidences for each row by performingphoton detection cyclically while shifting the read timing of each row.In this case, the amount of read timing shift of each row corresponds totime resolution. Accordingly, if the amount of shift is made small, thetime resolution of detection is improved in proportion to it.

6. Fifth Embodiment

FIG. 15 is a view showing an example of the configuration of a CMOSimage sensor (imaging device) according to a fifth embodiment of thepresent disclosure.

A CMOS image sensor (imaging device) 100D according to the fifthembodiment is different from the CMOS image sensor 100C according to thefourth embodiment shown in FIG. 13 in the following point.

In the fourth embodiment, the amount of shift is almost equal to theread time of one row. Also in the fifth embodiment, the time resolutioncan be improved even if the amount of shift is reduced without changingthe read time.

In the determination result integration circuit section 150D, twoadjacent sensing circuits 121-0 and 121-1 correspond to one row in thesensing circuit section 120D.

Corresponding to this, two adjacent 4-bit registers 211-0 and 211-1 ofthe first register section 210D correspond to one row.

In addition, in the second register section 220D, bit registers 222-0, .. . are arrayed corresponding to 4-bit registers.

Also in the fifth embodiment, the pixel block 160 (160-0, 160-1, . . . )is configured to include 128 digital pixels DPX and a selection circuitas in the fourth embodiment. The selection circuit selects one of thepixels to execute resetting or reading.

Also in the fifth embodiment, one pixel in the pixel block is selectedaccording to the row control line 180 driven by the row driving circuit170.

In addition, in the fifth embodiment, two circuits are prepared for eachcolumn for reading so that connections to different circuits arealternately made in odd and even rows.

For example, at the time of reading of a pixel DPX00, whether or notthere is an incidence of a photon on the selected pixel is output as anelectric signal to the output signal line 131-1 and the binary value isdetermined by the sensing circuit 121-0. For example, the sensingcircuit 121-0 determines “1” as a determination value if light isincident on the selected pixel and determines “0” as a determinationvalue if light is not incident on the selected pixel and latches thedetermination value. Then, the determination value of the sensingcircuit 121-0 is transmitted to the first bit of the 4-bit register211-0 of the first 4-bit register section 210D. Such reading is executedfor four rows.

On the other hand, at the time of reading of a pixel DPX01, whether ornot there is an incidence of a photon on the selected pixel is output asan electric signal to the output signal line 123-1 and the binary valueis determined by the sensing circuit 121-1. The determination value islatched by the sensing circuit 121-1 and is then transmitted to the4-bit register 211-1 at the next stage. Such reading is executed forfour rows.

After performing the above reading for four rows, the determinationvalues are simultaneously transferred to an eight-bit register 222 ofthe second register section 220D at the next stage. Then, the data heldin the 8-bit register 222 of each row is sequentially output to an 8-bitbus 230D and is then transmitted to the output circuit 240D. A countingcircuit 241D is disposed in the output circuit 240D in order to count oradd the data of “1” of each row. After all column data items of eightrows are transferred, the addition value of each row is stored in theoutput latch 242D.

Thus, the procedure of reading, transfer, and output is basically thesame as that in FIG. 14, but the read operation is divided according totwo lines of odd and even rows in this example.

These operations are executed in parallel while shifting the timing by ahalf period.

Here, time resolution of photon detection using the imaging deviceaccording to the fifth embodiment will be described with reference toFIG. 16.

FIG. 16 is a view for explaining the time resolution of photon detectionusing the imaging device according to the fifth embodiment. FIG. 16shows a state in which reading and resetting are sequentially executedfor each row according to the elapse of time.

By providing two read circuits in parallel, reading of the next row isstarted without waiting for the completion of reading of the previousrow. In addition, the shift of read time is a half period of the readperiod.

In a PET or the like, when γ-rays are incident on a scintillator, manyphotons are generated to be incident on corresponding imaging devices.Assuming that this timing is a dotted line 253, a photon is selectivelydetected only in row reading (expressed with thick oblique lines: RD) inwhich exposure time includes this time.

In this example, detection is performed until the row address takesnearly one round from reading immediately after the generation of aphoton (row address: 12), and then the data becomes zero. That is, if anoutput of row data of 1 or more occurs continuously or intermittentlyduring a period for which the row address takes a round, this is photongeneration.

Here, the total number of photons which are incident simultaneously onimaging devices is the total addition value of row data outputs for oneround. In addition, it can be estimated that the generation time is at254 between the read time of a row, in which an output of 1 or moreappears first, and the read time of a row before the row in which anoutput of 1 or more appears first.

The time resolution is a half period of a read period of one row, thatis, 125 nanoseconds.

Thus, it is possible to reduce the shift of the read period withoutshortening the read period itself. For example, it is also possible toreach the time resolution comparable to a photomultiplier tube byincreasing the number of reading systems further.

For example, in the case of an application to the PET, many imagingdevices according to the embodiment of the present disclosure arearrayed in a ring shape, and the system reads the number of photons ofeach row sequentially for every unit exposure for each imaging device.Then, when the generation of a photon is detected, the total number ofphotons incident simultaneously on the imaging devices and the timestamp of the generation are recorded on a memory. These are necessaryand sufficient data collected most efficiently.

By combining the data after finishing the imaging in order to identify apair of imaging devices, on which photons are incident simultaneously,the existence of an irradiated material can be assumed on the lineconnecting the pair.

Using this technique, it is possible to significantly increase thenumber of imaging devices itself compared with that in the related art.In addition, it is also possible to significantly extend the degree offreedom in combination of imaging devices on which simultaneousincidence of photons is to be determined. Accordingly, since thesensitivity can be greatly improved, the amount of medicine administeredcan be reduced significantly. As a result, it is possible to reduce theradiation exposure of a subject and also to improve the measurementaccuracy by suppressing accidental simultaneous generation of photons.

In addition, the solid state imaging devices according to the first andsecond embodiments described above may also be applied as imagingdevices of a digital camera or a video camera.

7. Sixth Embodiment

FIG. 17 is a view showing an example of the configuration of a camerasystem to which the imaging device according to the embodiment of thepresent disclosure is applied.

As shown in FIG. 17, a camera system 600 includes an imaging device 610to which a CMOS image sensor (imaging device) 100 according to thepresent embodiment can be applied.

The camera system 600 includes an optical system which guides lightincident on a pixel region of the imaging device 610 (forms a subjectimage), for example, a lens 620 which forms incident light (image light)on the imaging surface.

In addition, the camera system 600 includes a driving circuit (DRV) 630for driving the imaging device 610 and a signal processing circuit (PRC)640 for processing an output signal of the imaging device 610.

The driving circuit 630 includes a timing generator (not shown) whichgenerates various kinds of timing signals involving a start pulse or aclock pulse for driving circuits in the imaging device 610, and drivesthe imaging device 610 with a predetermined timing signal.

In addition, the signal processing circuit 640 performs predeterminedsignal processing on the output signal of the imaging device 610.

The image signal processed by the signal processing circuit 640 isrecorded on a recording medium, such as a memory. Hard copy of the imageinformation recorded on the recording medium is executed by a printer orthe like. In addition, the image signal processed by the signalprocessing circuit 640 is projected as a moving image on a monitorformed by a liquid crystal display or the like.

As described above, a high-precision camera which consumes low electricpower can be realized by providing the above-described solid stateimaging device 100 as the imaging device 610 in an imaging apparatus,such as a digital still camera.

In addition, although the configuration in FIG. 1 in which a pluralityof pixels share a sensing circuit is necessary when providing the pixelsand the sensing circuit on the same semiconductor substrate, a techniqueof forming a semiconductor layer with multiple layers using waferbonding technology has also appeared in recent years. In such a case,for example, a sensing circuit of each pixel may be provided in thelower layer of each pixel.

Also in this case, addition between pixels can be easily executed bymaking a plurality of sensing circuits share an integrated circuitincluding a counter. As a result, it is possible to improve the dynamicrange at the time of imaging.

The present disclosure contains subject matter related to that disclosedin Japanese Priority Patent Application JP 2010-224235 filed in theJapan Patent Office on Oct. 1, 2010, the entire content of which ishereby incorporated by reference.

It should be understood by those skilled in the art that variousmodifications, combinations, sub-combinations and alterations may occurdepending on design requirements and other factors insofar as they arewithin the scope of the appended claims or the equivalents thereof.

What is claimed is:
 1. An imaging device comprising: an imaging sectionwith at least one pixel that outputs an electric signal based on theincidence of a photon; sensing circuitry that performs a binarydetermination based on the electric signal from the pixel and outputs adetermination result; and determination result integration circuitrythat integrates the determination result of the sensing circuitry,wherein, the determination result integration circuitry derives theamount of incidences of photons on the pixel by counting photons, andthe photon counting is executed using a mesh comprising a lightreceiving surface divided into equal distances and a time axis dividedinto equal times.
 2. The imaging system of claim 1, wherein: the meshhas two logical values of logic 1 and logic 0, the sensing circuitrydetermines whether or not one or more photons have been incident on themesh and determines a logic 1 when a photon is incident regardless ofthe number of incident photons and determines a logic 0 when there is noincidence, and the determination result integration circuitry counts asum of 1 of the sensing circuitry.
 3. The imaging system of claim 1,further comprising: a plurality of pixel blocks, each comprising aplurality of pixels, selection circuitry for selecting among the pixels,and in the sensing circuitry, a respective sensing circuit disposedcorresponding to each of the pixel blocks.
 4. The imaging system ofclaim 3, wherein: the selection circuitry selects the pixels in eachpixel block in a cyclic manner and outputs a signal of a selected pixelto the respective sensing circuit, and the respective sensing circuitdetermines whether or not there is an incidence of a photon on eachpixel in a fixed period from last selection of that pixel to currentselection if that pixel.
 5. The imaging system of claim 4, wherein: areset function of resetting each of the pixels to a state where a photonis not incident is set, and an adjustment function of adjusting anexposure period by inserting reset processing between a selective outputof each pixel in the pixel block and a next selective output so that anexposure time in each pixel is fixed is set.
 6. An imaging devicecomprising: an imaging section with at least one pixel that outputs anelectric signal based on the incidence of a photon; sensing circuitrythat performs a binary determination based on the electric signal fromthe pixel and outputs a determination result; and determination resultintegration circuitry that integrates the determination result of thesensing circuitry; a counting circuit which performs the counting of thephotons; and a memory for storing a count result for each pixel in thecounting circuit, wherein, the plurality of sensing circuits share thecounting circuit for integrating the determination results.
 7. Animaging device comprising: an imaging section with at least one pixelthat outputs an electric signal based on the incidence of a photon;sensing circuitry that performs a binary determination based on theelectric signal from the pixel and outputs a determination result; anddetermination result integration circuitry that integrates thedetermination result of the sensing circuitry, wherein, thedetermination result integration circuitry derives the amount ofincidences of photons on the pixel by counting photons, thedetermination result integration circuit section outputs an additionvalue of photons incident for the row of pixels, and the determinationresult integration circuit section includes: (a) a register sectionincluding a line buffer which holds and outputs the determination valueof the sensing circuit for the row; (b) a bus through which output dataof the line buffer is transmitted; and (c) a counting circuit whichperforms count processing for integrating determination result data ofthe sensing circuit transmitted through the bus.
 8. An imaging devicecomprising: an imaging section with at least one pixel that outputs anelectric signal based on the incidence of a photon; sensing circuitrythat performs a binary determination based on the electric signal fromthe pixel and outputs a determination result; and determination resultintegration circuitry that integrates the determination result of thesensing circuitry, wherein, the determination result integrationcircuitry derives the amount of incidences of photons on the pixel bycounting photons, a reset function of resetting each of the pixels to astate where a photon is not incident is set, and the sensing circuitexecutes the binary determination by reading a signal in a reset stateand a read signal after exposure and adding an offset value to eitherone of the reset state and the read signal and comparing a signalobtained by addition of the offset value with the other signal.